NKX3.1 is a prostate-specific homeobox gene that maps to a region of chromosome 8p21 that is lost in up to 85% of prostate cancer cases. Although NKX3.1 does not undergo somatic mutations in prostate cancer, expression of the protein is lost with tumor progression, suggesting a role for NKX3.1 in prostate cancer pathogenesis. In Nkx3.1 mice haploinsufficiency is dominant, resulting in prostatic epithelial hyperplasia and dysplasia that worsens with age. Moreover, Nkx3.1 haploinsufficiency cooperates with loss of other suppressor genes such as Pten to enhance prostate carcinogenesis. These data suggest that loss of NKX3.1 expression may be important in pathogenesis of a large fraction of human prostate cancers and that NKX3.1 is a candidate gatekeeper gene. We described an NKX3.1 polymorphism, C154T, that resulted in an arginine to cysteine alteration of codon 52 (NKX3.1 R52C). In the initial grant period we showed that a single NKX3.1 C154T allele, present in 11% of the population, conferred an increased risk for aggressive prostate cancer. We also showed that the R52C variant altered phosphorylation at the adjacent serine 48 (S48) and that S48 phosphorylation regulated DNA binding in vitro. But NKX3.1 influences gene expression not only by DNA binding, but also by complexing with transcription factors and regulating their activity as a coactivator. Preliminary data shows that the region of amino acids 44-64 is critical for autoregulation of NKX3.1 coactivation activity, presumably by mediating binding to the C-terminus. We now will determine biochemical properties of NKX3.1 critical for its action. In Aim 1 we will perform genetic analysis of NKX3.1 to identify critical elements that regulate protein activity. In Aim 2 we will perform affinity chromatography with NKX3.1 to isolate and identify proteins that bind to NKX3.1. In Aim 3 we will identify and characterize genes whose expression is regulated by NKX3.1. In Aim 4, we will analyze tumor specimens from patients with high-grade prostate cancer to determine whether in those with the C154T polymorphic allele it is preferentially retained after loss of chromosome 8p heterozygosity.